This invention relates to an engine ignition system and more particularly to an engine ignition system for changing the ignition timing of an engine in accordance with a throttle opening degree.
FIG. 1 is a circuit diagram of a conventional engine ignition system. In the figure, reference numeral 1 is a generator coil of a magnet generator driven by an unillustrated engine. One end of the generator coil 1 is connected to an ignition capacitor 3 through a diode 2 and the other end is grounded. 4 is a thyristor for allowing the discharge of the ignition capacitor 3 into an ignition coil 5, 6 and 7 are a primary coil and a secondary coil, respectively, of the ignition coil 5 and the secondary coil 7 is connected to an ignition plug 8. 9 is a diode connected in parallel to the primary coil 6 for bypassing a reverse electromotive force across the ignition coil 5, and 10 is a bias resistor connected between the gate and the cathode electrodes of the thyristor 4.
11 is a power source circuit composed of a series circuit including a diode 12, a resistor 13 and a Zener diode 14 connected across terminals of the generator coil 1 and a capacitor 15 connected in parallel to the Zener diode 14 and having an output terminal at a junction between the resistor 13 and the Zener diode 14.
16 is a signal coil generating an ignition signal in synchronization with the rotation of the engine, one end of which is connected to the gate of the thyristor 4 through a resistor 17 and a diode 18 and grounded through a diode 19 in the opposite direction and a resistor 20, and the other end of which is grounded. 21 is an ignition timing calculation circuit having its input terminal connected at a junction between the diode 19 and the resistor 20. The ignition timing calculation circuit calculates an ignition timing in accordance with an output signal from the signal coil 16 and a signal from a throttle sensor 22 which detects the degree of opening of the throttle, thereby providing an ignition timing control signal to the gate of the thyristor 4. At the junction between the resistor 17 and the diode 18, the collector of a transistor 23 is connected. The base of this transistor 23 is connected to a junction between a resistor 25 connected in series to the power source and a capacitor 26, and the emitter is connected to ground.
The operation will now be described. When the engine is driven by an unillustrated starter or the like, the generator coil 1 generates an output power, which charges the capacitor 3 after it is rectified by the diode 2. The signal coil 16 on the other hand generates the ignition signal in synchronization with the rotation of the engine. A positive wave component or retarded angle side reference signal is directly applied to the gate of the thyristor 4 through the resistor 17 and the diode 18, and the negative wave component or the advanced angle side reference signal is supplied to the ignition timing calculating circuit 21, which calculates an ignition timing corresponding to the throttle opening degree detected by the throttle sensor 22 on the basis of the advanced angle side reference signal. This ignition timing calculation is set to provide a retarded angle when the throttle opening degree is closure side and an advanced angle when the throttle opening degree is open side, providing the ignition timing control signal corresponding to the throttle opening degree to the gate of the thyristor 4. The thyristor 4 is brought into conducting state by the retarded angle side reference signal or the ignition timing control signal to cause the charge on the capacitor 3 to discharge through the primary coil 6 of the ignition coil 5, thereby generating a high voltage at the secondary coil 7 to generate a spark across the spark plug 8.
At the time when the engine is being started, the output voltage from the generating coil 1 is low and the output voltage from the power source circuit 11 does not reach the predetermined level, maintaining the transistor 23 in the nonconducting state, so that the retarded angle side reference signal is directly applied to the gate of the thyristor 4. After a lapse of a predetermined time after starting of the engine, the voltage at the junction between the resistor 25 and the capacitor 26 reaches the predetermined level, causing the transistor 23 to turn on to bypass the retarded angle side reference signal through the transistor 23. That is, during the starting operation of the engine, the ignition operation is achieved by the retarded angle side reference signal, and during the normal operation after starting of the engine, the ignition operation is achieved by the ignition timing control signal which corresponds to the degree of opening of the throttle.
In the conventional engine ignition system constructed as above described, the ignition timing calculating circuit 21 calculates the ignition timing for the next ignition by the advanced angle side reference signal from the signal coil 16. Therefore, at around the throttle position of complete closure, the time between the input of the reference signal and the output of the ignition timing control signal is elongated, so that the ignition timing at low rpms becomes unstable, causing the rotation to be disadvantageously unstable.